My WordPress Development Wishlist
Standardize saving post meta attributes. The state of
save_postis a mess. Is autosave, has nonce, has permissions, is cron, ir WP-API? See https://github.com/xwp/wp-safe-input
Introduce templating for building configuration screens in the admin. Probably as an abstraction over the Settings API that is actually friendlier and easier to use. Decouple data store from where and how the settings are presented to a user.